Minimization in integrated circuit (IC) dimension allows faster device speed, higher device packing density, and the integration of more functions on a single chip. However, the propagation delays increase with increasing numbers of interconnects, and limits the overall performance of the device. From a materials point of view, in order to lower the propagation delay, higher conductivity metallization and lower dielectric constant (k) materials are required to replace the Al and SiO2/polyimide interconnect structure [1]. Although Cu has successfully replaced Al and become the current metallization material, designing low-k dielectrics remains one of the main challenges today.
Development of low-k materials has lagged behind other areas of semiconductor R&D [2]. As early as 1999, the Internal Technology Roadmap for Semiconductors (ITRS) called for materials having a dielectric constant of 2.2, a goal that was modified several times and ultimately pushed out to 2007 [3]. The reason for this lag is that interlayer dielectric (ILD) materials need to meet stringent material property requirements for successful integration into the structures. These include sufficient mechanical properties to survive chemical mechanical polishing (CMP) processing. Moreover, properties, such as Young's modulus, hardness, and thermal conductivity decrease with increasing porosity, which is used to achieve lower k values [4].
In April 2000, IBM announced its intent to use SiLK™ (Dow Chemical Co.) [5], a spin-on aromatic thermosetting polyphenylene. By 2003, IBM considered abandoning SiLK™, because of its low mechanical properties (Young's modulus of 2.45 GPa and hardness of less than 0.38 GPa [5]) and mismatch in coefficient of thermal expansion with both metal and ceramic substrates [6]. In February 2004, Applied Materials held a press conference to trumpet the use of its Black Diamond™ low-k CVD film by several firms in 90 and 130 nm critical dimension semiconductors [7]. Black Diamond™ is derived from plasma polymerization of siloxane and has a general composition of SiOxCyHz. It has a dielectic constant lower than 2.7, an elastic modulus of 3.5 GPa and a hardness greater than 1.5 GPa. During the conference, Farhad Moghadam pointed out that a key property of the material is the hardness, which is greater than 1.5 GPa. However, the extendability of Black diamond™ to next generation materials (k<2.4) is questionable.
Aromatic polydiacetylenes appeared to be good low-k candidates because they are known to have a relatively low k value, low moisture uptake, excellent thermal stability, and a curing reaction which proceeds without evolving volatiles. Poly(m-diethynyl benzene) (polyDEB) was initially discovered in the early 1960s by Hay and co-workers [11]. The high molecular weight polymer was prepared by the oxidative coupling of m-diethynyl benzene. The polymer undergoes an apparent exothermic decomposition upon heating to 180° C. and showed a weight retention of over 90% after heating to 800° C. under inert conditions [12]. Alternatively, the decomposition observed at 180° C. could have been the result of an uncontrolled exothermic curing process. PolyDEB is an aromatic polydiacetylene, having butadiynylene groups (—C≡C—C≡C—) along the polymer backbone. In 1969, Wegner [13] reported that monomeric diacetylenic single crystals could be polymerized by ultraviolet radiation. This finding suggested that polymers containing diacetylene units would have potential as photoresists.
Poly(p-diethynyl benzene) (polyPEB) typically has high crystallinity and is difficult to dissolve in common solvents. A cured thin film of polyPEB is very brittle [14]. Economy et al. [15–16] developed a highly soluble poly(triethynylbenzene) (polyTEB) as a thermally stable insulating thin film for electronic applications. However, its poor adhesion, huge exotherm during curing, brittleness and high cost limited further applications. TEB oligomers synthesized and formed into films according to their methods broke into small pieces during thermal curing even though the film thickness was below 1 μm. A thinner film (less than 500 nm) could be formed by applying an adhesion promoter. UV pretreatment could be used to further reduce the exotherm during thermal curing. DEB has better solubility and less rigidity than PEB, as well as a lower exotherm and cost compared to TEB. However, its solubility in common organic solvents is still low for use in spin-on coating techniques and it has a tendency to crack prior to curing, resulting in poor film formability [17–18]. Several studies have been carried out to solve the low solubility problem of polyDEB. Stille and Whitesides [19–21] made soluble poly(diacetylene) by incorporating a solubilizing aliphatic side group. Miller et al. [22] synthesized several copolymers containing aromatic diacetylenes and showed the dielectric constants for these polymers were around 2.82 to 3.34 with low moisture uptakes, good film formability and photo definition.